Adsorption characteristics of CO₂ and CH₄ on dry and wet coal from subcritical to supercritical conditions

Enhanced coal bed methane (ECBM) recovery has been proposed as an attractive way to store captured CO₂ while recovering CH₄. The adsorption and desorption behaviors of CO₂ and CH₄ on dry and wet coal (anthracite) were studied at 318 and 338K and up to 150atm. The sorption capacity of CO₂ and CH₄ on anthracite coal was higher at lower temperatures and dry coal condition, but smaller than those on bituminous coals at a similar condition. In wet coal, the sorption capacity and stability of high pressure CO₂ stored at 318K could be lower than those at 338K in the supercritical region because higher density of CO₂ at 318K could lead to the structural change of wet coal. The difference in the excess adsorbed amount between dry and wet coal was only noticeable under the subcritical conditions at 338K but became more significant under the supercritical conditions with pressure at 318K. In dry and wet coal, the CO₂ desorption isotherms had different shapes, depending on temperature, but all the CH₄ desorption isotherms showed a weak positive hysteresis. The mutual solubility between the CO₂-rich (or CH₄-rich) phase and aqueous phase as well as coal swelling should be considered in evaluating the sorption capacity of a wet coal seam. Fluid density in free volume was the important variable to estimate the CO₂ storage capacity or ECBM recovery because the density variation significantly influenced the isotherm shape.